Leaders of the major unions welcomed the outcome. Tony Woodley of the Unite union said: "This is a fantastic achievement for him and for the policies he has been promoting.
"His victory, coming from nowhere a few months ago, is a clear sign that the party wants change."
But one senior MP said it was bleak day. "I think this will trigger a constitutional crisis in the party. It is complete madness that we can be seen to have a leader who was put there by the unions," the MP said.